A Sponsor for the Message. Currently, we only support attaching a single sponsor when posting to only Facebook verified Pages (a.k.a. Facebook Branded Content, see https://www.facebook.com/facebookmedia/get-started/branded-content for more info), so any attempt to use it either for other Social Networks or including targets that don't comply with this rule, will result in an error from the back-end. Note that the Sponsor itself is not restricted to be a verified page, so it could be a regular (non-verified) page or a regular user. As an example, a post could be created, targeting the Facebook verified page of @LeoMessi and that post could be sponsored by @nike.